The invention concerns bearing reinforcements for shafts that are mounted on bearings in a cast light-metal housing, e.g. an engine block, gearbox housing or the equivalent, in which inserts that form the bearing seats for the shafts are cast in the light-metal housing, wherein the inserts are made of an MMC (Metal Matrix Composite) material of a type such that the base metal of the MMC material is the same as the base metal that constitutes the principal component in the material that forms the light-metal housing.
The engine blocks and cylinder heads in modern combustion engines are often made of aluminum alloys in order to save weight. The difference in terms of thermal expansion between Al alloy, ca. 22 ppm/xc2x0 K. in an engine block, and steel, ca. 12 ppm/xc2x0 K. in, e.g. crankshafts mounted in the main bearings in the engine block entails disadvantages in terms of the ability to control the radial play present in the crankshaft bearings. Because the bearing seat in the Al block contracts more at low temperatures than the steel shaft, sufficient radial play must be present to prevent the shaft from seizing. This play increases at operating temperature of the engine. It can lead to difficulties in maintaining a proper lubricant film at the bearing within the entire temperature range of the engine, i.e. xe2x88x9230xc2x0 C. to +150xc2x0 C. The increased radial play at higher temperatures can also cause crankshaft vibrations. In the present state of the art, this problem is addressed by casting inserts of iron-based material in the block at the bearing seats. However, this approach is encumbered by problems in the form of, e.g.:
insufficient binding between the different materials,
dissimilarities in terms of workability,
incompatibility in connection with reuse.
The temperatures are high beneath the hood of a modern car, and once the engine has reached its operating temperature the transmission can also be subjected to temperatures ranging from xe2x88x9230xc2x0 C. to +150xc2x0 C. To save weight, aluminum alloys are being used to an ever-increasing extent in gearbox housings and other similar components. Because of the high thermal expansion exhibited by these aluminum alloys, this large operating temperature range can lead to design problems in connection with gear play (play between tooth gear flanks) and grip in rolling-element bearings in the gearbox (refers to the grip between housing and bearing that holds the bearing in place). At low temperatures, the center distances between the shafts arranged in the gearbox are low, which requires that there be sufficient gear play between the drive gears on two different shafts when these drive gears are brought into mutual engagement. However, there cannot be so much play between the gears that it results in noises and abrasion at high operating temperatures, when the center distance between two such shafts is greatest.
In a similar way, the rolling-element bearings for the shafts in the gearbox housing require sufficient play at low temperature while, at the same time, the amount of play present must not be excessive at the operating temperature of the gearbox. This is difficult to achieve while simultaneously meeting the requirement that the bearings maintain their grip in the housing throughout the entire temperature range. There is no known solution to this problem. This problem can be expected to become worse as the temperatures under vehicle hoods become higher in the future, at the same time as heavier demands are made in terms of transmission efficiency. The demand for lower vehicle fuel consumption provides the impetus in both cases.
The use of MMC material is proposed in the invention described below. Commercial interest in structural materials of the type known generally as MMCs (xe2x80x9cmetall-matris-kompositerxe2x80x9d in Swedish) increased during the 1990s. MMC materials are composites that consist of a binding material in the form of a metal, usually a light metal such as aluminum, magnesium, titanium or alloys thereof, which is reinforced via the admixture of fibers or particles in the form of ceramic substances such as silicon carbide, boron carbide or aluminum oxide. MMCs can also consist of steel produced by power metallurgy and reinforced with titanium carbide. MMC materials exhibit extremely interesting properties, which can be further adapted depending on the area of application, thereby offering advantages in that the component can be made lighter, stronger, stiffer and possessing better fatigue properties than can be achieved using conventional materials within the specific area of application in question.
One significant disadvantage associated with the use of MMC materials is that they are very difficult to machine. To create a component made of MMC material, methods are usually used in which the component is cast in a shape that is closely akin to the final shape of the component. Another method involves using a forged billet or a piece of extruded bar, so that spark machining of the surface and conventional cutting techniques can be used to produce the final component shape. Attempts have been made to produce e.g. connecting rods for motorcycles by means of such conventional fabricating and machining methods. In this way, the goal of achieving the desired component and its desired properties, such as lower weight, has been achieved, and the use of such connecting rods in an engine has resulted in an engine that turns more readily and vibrates less. However, the problem is that the cost of producing engine components by conventional means has been extremely high, thereby limiting use to areas where cost is of lesser importance.
A number of patents document various methods for the final forming of components based on MMC materials. U.S. Pat. No. 5,765,667 may be cited as an example of such a patent, wherein a method is described for fabricating a component, in this case a brake disk, by casting to a format that is as close as possible to the shape of the finished component in order, as is clearly described, to avoid the need for machine cutting to the greatest possible extent. It is obvious to one skilled in the art to avoid the need for cutting operations, since the MMC material contains, when consisting of e.g. an aluminum base and reinforcing particles of silicon carbide, the very substances that are commonly used to grind cutting tools. The silicon carbide particles imbedded in the MMC material have a destructive effect on cutting tools when conventional cutting methods are used, since the edges of the cutting tools are rapidly worn down by the abrasive particles in the composite material.
Patent application PCT/SE/02007, which was not yet published at the time of the submission of the present application, presents a method that shows that MMC materials can be machined by HSM (High-Speed Machining), and this method was used in the fabrication of products related to the present invention. Everything described in patent application PCT/SE/02007 is hereby incorporated into the present patent application.
Patent application PCT/SE/02007, which was not yet published at the time of the submission of this application, presents a method that shows that MMC materials can be machined by HSM (High-Speed Machining), and this method was, among others, used in the fabrication of products related to the present invention. Everything described in patent application PCT/SE/02007 is hereby incorporated into the present patent application.
One aspect of the invention pertains to a bearing reinforcement for a shaft that is mounted in a cast light-metal housing, such as an engine block, a gearbox housing or the equivalent, in which inserts that form the bearing seats for the shafts are cast in the light-metal housing, wherein the inserts are made of an MMC (Metal Matrix Composite) material of a type such that the base metal of the MMC material is the same as the base metal that constitutes the principal component in the material that forms the light-metal housing.
According to another aspect of the invention, a method is described for fabricating an insert made of MMC material for casting in a light-metal housing to form a bearing seat and thus achieve a bearing reinforcement as per the foregoing in which the inserts are created by extruding a billet of MMC material to produce a bar with a profile whose cross-section essentially corresponds to that of the finished insert. The inserts are then cross-cut from the extruded bar by cutting via high-speed machining, HSM, as defined below and in patent application PCT/SE/02007. Other methods, such as water cutting, can also be used to cross-cut the bar material into separate inserts.
Using an aluminum-based metal matrix composite, MMC, in cast inserts for the bearing seats in an engine block made of aluminum alloy, it is possible to achieve thermal expansion similar to that of the steel used in the shaft at the bearing seat with materials that match the material in the engine block in terms of compatibility during casting, future reuse of the engine block, and that can be machined using similar techniques.
Additional advantages afforded by using MMC materials at the aforementioned points consist in that their high modulus of elasticity contributes to a rigid design that retains the low weight of aluminum. High abrasion resistance reduces damage from erosion between the insert and the outer surface of the bearing casing.
The described technique is naturally applicable to bearing seats in light-metal housings for other types of shafts, such as balance shafts and camshafts in engine blocks, or shafts in gearboxes.
Yet another advantage of using cast inserts made of MMC material as bearing reinforcements in light-metal housings as per the foregoing is that the position of the shaft center line is maintained in that the play between shaft and bearing housing is reduced. This also extends the useful life of the oil used to lubricate the bearings.
One means of reducing fuel consumption in future vehicles is to replace aluminum with magnesium as the principal component in light-metal housings in combustion engines and gearboxes. The fact that the coefficient of linear expansion for magnesium is even higher than for aluminum, and that the modulus of elasticity for magnesium is much lower than for aluminum, provides even greater justification for use MMC inserts as bearing reinforcements in this case. In fact, the use of MMC inserts according to this aspect of the invention could be what makes it possible to use magnesium-based light metals in engine blocks for combustion engines, and in gearbox housings.
The foregoing assertions concerning the use of MMC inserts apply to all types of means of conveyance, i.e. not only within the automobile industry, but to an equally great extent in e.g. airplanes and helicopters.